∵x＞0，
∴y=3x+4x≥212=43
当且仅当3x=4x时，等号成立，
故选B．
20多年前，澳大利亚从我国引进并放养一种动物，用于治理草原上堆积的牛粪，这种动物在恢复和保护牧场、调节生态平衡中起了重大作用。它是（ ）
A羊 B.屎壳郎 C.蚯蚓 D.野兔
Which of the following functions would the nurse expect to be unrelated to the placenta
A．Production of estrogen and progesterone.
B．Detoxification of some drugs and chemicals.
C．Exchange site for food, gases, and waste.
D．Production of maternal antibodies.
